MM-41866: Adds index to UserGroups.DisplayName. (#19799)

db/migrations/postgres/000079_usergroups_displayname_index.down.sql

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+DROP INDEX IF EXISTS idx_usergroups_displayname;

db/migrations/postgres/000079_usergroups_displayname_index.up.sql

@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS idx_usergroups_displayname ON usergroups(displayname);